Aluminum/0.25-4% copper alloys have become the metallization of choice for very large scale integrated circuits (VLSI). The most common methods for depositing the alloy are magnetron sputtering or other physical vapor deposition (PVD) techniques. However, as the feature size of ultra-large scale integrated circuits (ULSI) continues to decrease to the sub-0.5 .mu.m regime, it becomes increasingly difficult to successfully use PVD metallization. PVD is largely a line-of-sight technique and has difficulty conformally coating &lt;0.5 .mu.m high aspect ratio features. This limitation of PVD has created a growing interest in chemical vapor deposition (CVD) techniques. CVD is an inherently conformal process and excellent step coverage is possible. Also, since CVD can be catalyzed by portions of the substrate surface, selective deposition is also possible.
State-of-the-art integrated circuits, such as Intel's 586 (i.e. Pentium.TM.) processor, uses CVD tungsten for filling vias because of inadequate step coverage with PVD techniques. However, CVD tungsten, while having excellent step coverage, has a much higher resistivity than Al/(0.25-4% Cu) (10-15 vs. 3.mu..OMEGA.-cm, respectively). The resistivity of the CVD tungsten metallization can limit the clock speed of the device due to the RC-time delay. This effect becomes more important as the minimum feature size of integrated circuits continues to decrease and the clock speed continues to increase. Thus, for future generations of integrated circuits, high resistivity CVD tungsten will have to be replaced with a lower resistivity CVD metal, such as Al/(0.25-4% Cu) alloy. However, currently, there is no good process for depositing Al/(0.25-4% Cu) by CVD codeposition.
There exists a need in the electronics industry for a new process for depositing Al/(0.25-4% Cu) alloy metallization by CVD co-deposition. Sputtered Al/(0.25-4% Cu) is used for metal runners. The metal runners are not high aspect ratio features and sputtering gives adequate step coverage. The advantages of using CVD Al/(0.25-4% Cu) for via fills instead of CVD tungsten are, the same alloy is used for all metallization steps, and CVD Al/(0.25-4% Cu) has a very low resistivity. Using the same alloy for all metallization steps should dramatically simplify processing. The advantages are numerous; there would be no need for diffusion barriers and adhesion promoters between the via fill and the runners, thus reducing cost; aluminum based alloys are easily etched to define the circuit lines; Al/(0.25-4% Cu) has a much lower resistivity than CVD tungsten, therefore a much lower RC-time delay constant.
Dimethylethylaminealane, DMEAAl, has previously been shown to be the most effective available precursor for the chemical vapor deposition of pure aluminum metallization at temperature greater than about 150.degree. C. The compound has been described in U.S. Pat. No. 5,191,099. Trimethylaminealane (TMAAl) has also been used effectively to deposit high quality aluminum, as described in U.S. Pat. No. 4,923,717. However, to date, the utility of the DMEAAl precursor has been limited by the lack of a suitable precursor and process to co-deposit copper with DMEAAl. While the CVD of pure aluminum has been known for many years, pure aluminum does not have sufficient electromigration resistance to be useful for ULSI interconnects. Therefore, there is a need in the electronics industry for a process to co-deposit copper and aluminum using DMEAAl and a volatile copper precursor. The difficulty in developing a Cu/Al codeposition process is that the requirements for an appropriate copper precursor are very stringent. The copper precursor must be:
1. Sufficiently volatile at &lt;100.degree. C. PA1 2. Thermally stable at room temperature for extended periods of time. PA1 3. Chemically compatible with DMEAAl, as well as other available aluminum precursors. PA1 4. Not suffer from parasitic pre-reactions with DMEAAl or the aluminum precursor of choice. PA1 5. Not contain oxygen, fluorine, primary amine hydrogen, or phosphorus. The high reactivity of DMEAAl and other alanes with oxygen and fluorine requires that the copper precursor not contain any of these elements. Phosphorus is a n-type dopant for silicon and is therefore an unacceptable constituent for a CVD precursor. Primary amine hydrogens react with the hydride of the DMEAAl or other alanes forming an aluminum-amide polymeric species. PA1 6. The co-deposition process with DMEAAl or other aluminum precursor must deposit Al/(0.25-4% Cu) which has a uniform composition, low resistivity, smooth morphology, small grain size, and does not contain chemical impurities.
Only recently has research focused on techniques to incorporate copper into aluminum films. Several two step processes have been reported. Kwakman, Cheung and coworkers report copper incorporation into CVD aluminum by sputtering copper onto the substrate either before or after CVD aluminum deposition, see K. P. Cheung, C. J. Case, R. Liu, R. J. Schutz, R. S. Wagner, L. F. Tz. Kwakman, D. Huibregtse, H. W. Piekaar, and E. H. A. Granneman in Proceedings of the International VLSI Multilevel Interconnection Conference, (IEEE, 1990), p. 303, and L. F. Tz. Kwakman, D. Huibregtse, H. W. Piekaar, and E. H. A. Granneman, K. P. Cheung, C. J. Case, Y. Lai, R..Liu, R. J. Schutz, R. S. Wagner, ibid., p. 282. Fine et. al. report a two-step consecutive selective deposition using trimethylaminealane (TMAAl) and (hfac)Cu(tmvs) in S. M. Fine, P. N. Dyer, J. A. T. Norman, in Chemical Perspectives of Microelectronic Materials II, (Mater. Res. Soc. Proc. 204, Pittsburgh, Pa., 1990) pp.415-420. U.S. Pat. No. 5,273,775 discloses such a process. U.S. Pat. No. 5,085,731 discloses analogous copper ketone compounds and their synthesis. U.S. Pat. No. 5,098,516 describes the use of such compounds to form discrete copper layers or films.
Previous attempts to develop CVD codeposition of aluminum/copper have been unsuccessful due to either incompatibility of the copper and aluminum precursors or thermal incompatibility of the copper precursor (either too reactive or unreactive at useful process temperatures). Houlding et. al. report a copper/aluminum codeposition technique in V. H. Houlding, H. Maxwell, Jr., S. M. Crochiere, D. L. Farrington, R. S. Rai, and J. M. Tartaglia (Materials Research Society Proceedings 260, Pittsburgh, Pa. 1992) pp 119-124. However, their process has problems with parasitic prereaction between the copper and aluminum precursors and thermal instability of the copper precursor. Katagiri et. al. report the CVD codeposition of Al/(0.25-4% Cu) alloy by CVD codeposition of dimethylaluminum hydride, DMAH, and (cyclopentadienyl)copper(triethylphosphine), CpCu(PEt.sub.3), see Katagiri, E. Kondoh, N. Takeyasu, and T. Nakano, Jpn. J. Appl. Phys., 32, L1078, (1993) and E. Kondoh, Y. Kawano, N. Takeyasu, and T. Ohta, J. Electrochem. Soc,, 141, 3494, (1994). The difficulties with this process are the thermal instability, and presence of phosphorus in CpCu(PEt.sub.3), and the tendency of DMAH to oligomerize into high molecular weight and viscous dimethylaluminum hydride polymers. Formation of these polymers in the DMAH bubbler will cause inconsistent delivery rates for the aluminum precursor. A potentially chemically compatible copper compound, [Cu[N(SiMe.sub.3).sub.2 ]].sub.4, is known to be stable to at least 180.degree. C., as reported by Burger, H., and Wannagut, U., Monatsh., 95, 1099 (1964), making it unacceptable for low temperature CVD applications.
A class of copper .beta.-diketimine compounds (Cu(BDI).sub.2) have been reported in the literature, S. G. McGeachin, Can. J. Chem., 46, 1903, (1968), but a utility for their use in accordance with the present invention was not contemplated for them.
The drawbacks of the prior art of multiple film formation steps, precursor compounds which are difficult to handle, and temperatures of film forming which are inappropriately high are overcome by the present invention as set forth below.